1. Field of the Invention
This invention relates to a method and apparatus used in, for example, a digital oscilloscope for detecting occurrence of aliasing.
2. Description of the Related Art
A prior art aliasing detecting apparatus of this kind used in, for example, a digital oscilloscope is disclosed in JP-A-59-48658. The term "aliasing" referred to above designates the phenomenon that occurs when the frequency of a sampling pulse signal (clock) A is less than two times as much as that of an analog input signal S, as shown in FIG. 6. It will be seen in FIG. 6 that, after D/A conversion of the sampled data P again, the waveform of the reproduced analog signal S' has a frequency lower than the actual frequency of the input signal S.
FIG. 5 is a block diagram schematically showing the structure of the prior art aliasing detecting apparatus. Referring to FIG. 5, the prior art aliasing detecting apparatus includes a sampling pulse counter 1, a reset pulse generator 2, an input signal cycle counter 3 and an error detector 4. The sampling pulse counter 1 divides the frequency of a sampling pulse A thereby generating a sampling pulse count signal B. The reset pulse generator 2 detects the rising edge or falling edge of the sampling pulse count signal B applied from the sampling pulse counter 1 thereby generating a reset pulse C. The input signal cycle counter 3 generates an error detection signal E when a signal D obtained by converting an analog input signal into a binary signal rises or falls two or more times within a predetermined period of time. The error detector 4 displays occurrence of aliasing on an external display unit such as an LED. In the prior art apparatus, the reset pulse generator 2 generates one reset pulse C each time it receives three sampling pulses. The fact that the signal D rises or falls two or more times within the above period of time means that the frequency of the input signal exceeds 1/2 of the sampling frequency, and undesirable aliasing is occurring. When such a situation is detected, an alarm is issued to the user of, for example, a digital oscilloscope so as to prevent wrong operation of the digital oscilloscope.
However, the prior art aliasing detecting apparatus has had the problem that, because the sampling period counter and the reset pulse generator are essentially required, the circuit scale is inevitably correspondingly increased. Another problem of the prior art aliasing detecting apparatus is how to convert its input signal into a binary signal.